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When it comes to caffeine, all coffees are not created equal. According to a recent report from the Center for Science in the Public Interest, popular brands varied widely when it comes to the jolt they provided. McDonald’s, for instance, had 9.1mg per fluid ounce, while Starbucks packed more than double that at a full 20.6mg. For more on those findings, click here.

It’s not just coffee beans: Tea leaves, kola nuts (which flavor colas), and cocoa beans all contain caffeine. The stimulant is found naturally in the leaves, seeds, and fruits of a wide variety of plants. It can also be man-made and added to products.

A strong, rich flavor might seem to indicate an extra dose of caffeine, but the truth is that light roasts actually pack more of a jolt than dark roasts. The process of roasting burns off caffeine, NPR reports, meaning those looking for a less intense buzz might want to opt for the dark roast java at the coffee shop.

By definition, one might reasonably think that energy drinks would pack loads of caffeine. But many popular brands actually contain considerably less than an old-fashioned cup of black coffee. An 8.4-ounce serving of Red Bull, for instance, has a relatively modest 76 to 80mg of caffeine, compared to the 95 to 200mg in a typical cup of coffee, the Mayo Clinic reports. What many energy drink brands frequently do have, though, is tons of sugar and hard-to-pronounce ingredients, so it’s best to stay clear of them anyway.

The body might process caffeine differently based on gender, race, and even birth control use. New York magazine previously reported: “Women generally metabolize caffeine faster than men. Smokers process it twice as quickly as nonsmokers do. Women taking birth control pills metabolize it at perhaps one-third the rate that women not on the Pill do. Asians may do so more slowly than people of other races.”

In The World of Caffeine: The Science and Culture of the World’s Most Popular Drug, authors Bennett Alan Weinberg and Bonnie K. Bealer hypothesize that a nonsmoking Japanese man drinking his coffee with an alcoholic beverage—another slowing agent—would likely feel caffeinated “about five times longer than an Englishwoman who smoked cigarettes but did not drink or use oral contraceptives.”

According to the American Academy of Sleep Medicine, it takes about 30 to 60 minutes for caffeine to reach its peak level in the blood (one study found increased alertness can begin in as few as 10 minutes). The body typically eliminates half of the drug in three to five hours, and the remainder can linger for eight to 14 hours. Some people, particularly those who don’t regularly consume caffeine, are more sensitive to the effects than others.

Sleep experts often recommend abstaining from caffeine at least eight hours before bedtime to avoid wakefulness at night.

Most of us consume it every day, but how much do we really know about caffeine? The naturally-occurring substance with a bitter taste stimulates the central nervous system, making you feel more alert. In moderate doses, it can actually offer health benefits, including boosts to memory, concentration, and mental health. And coffee in particular, a major source of caffeine for Americans, has been associated with a host of body perks, including a possible decreased risk of alzheimer’s disease and certain cancers.

But in excess amounts, caffeine overuse can trigger a fast heart rate, insomnia, anxiety, and restlessness, among other side effects. Abruptly stopping use can lead to symptoms of withdrawal, including headaches and irritability.

1 of 10 Decaf Isn’t the Same as Caffeine Free

Think switching to decaf in the afternoon means you aren’t getting any of the stimulant? Think again. One Journal of Analytical Toxicology report looked at nine different types of decaffeinated coffee and determined that all but one contained caffeine. The dose ranged from 8.6mg to 13.9mg. (A generic brewed cup of regular coffee typically contains between 95 and 200mg, as a point of comparison. A 12-ounce can of Coke contains between 30 and 35mg, according to the Mayo Clinic.)

“If someone drinks five to 10 cups of decaffeinated coffee, the dose of caffeine could easily reach the level present in a cup or two of caffeinated coffee,” says study co-author Bruce Goldberger, Ph.D., a professor and director of UF’s William R. Maples Center for Forensic Medicine. “This could be a concern for people who are advised to cut their caffeine intake, such as those with kidney disease or anxiety disorders.”